How do I make a round cornered rectangular box with a domed top - for 3D printing
A quick, destructive approach:


Starting in the flat, a plane subdivided along the convex dimension
O proportional editing, spherical profile,  G Z raise the central edge / face.
Subdivide in the other direction
E Z extrude the perimeter edges downward, and  S Z0 flatten them
Ctrl B with  a few segments, bevel the sharp edges.
Maybe scale some edge loops inwards, to ease the curvature in the vicinity of the bevel


Solidify and Subdivision Surface modifiers,  for thickness, and smoothed high-resolution, for printing.